Im going to tell you an anectdote, than give you a contradictory theoretical response.
Anecdote. I friend my mine wanted Uro. He signed up for three aways. After the first, he realized that "how crap, this **** is hard..." and didn't want to do three straight months. He told the THIRD place that he just wasn't going to do it. They had 2 months notice, a long waiting list, and certainly didn't need him. He did not get an interview at the place he canceled, but matched his first choice, the place where he DID do an away.
Theory. Medicine is a really small world. There just aren't enough people in this field to not have something come back and bite you. Is it ok to give them advanced notice, saying "nvm, my b?" Sure it is. But just remember, you aren't going to get an interview there, and probably, SOMEONE will remember you as the jack ass who bailed on them. Its impact? Maybe its nothing. But maybe the person who feels burned happens to have a lot of PD friends, who, not being malicious, talks poorly about you to his friends. Maybe one day that guy is the president of AMA and you want a political position, but he remembers you.
Unlikely, right? yeah. Chances are the impact is minimal; you won't get an interview at that ONCE place, and then it will never again hurt you. But unless you've got a good reason not to, why not just do it?
Ultimately, its up to what you want. As the anecdote shows, it didn't hurt that guy too much. Chances that it hurts you in the long run? Slim. I've learned that any of those slim chances you can actually control, you should control, and probably avoid (by just doing the away).